1. Field of the Invention
The invention relates to an ultrasonic diagnostic imaging system that uses nonlinear distortion for imaging tissue inside a body.
2. Description of the Prior Art
Ultrasonic diagnostic imaging systems for imaging tissue inside a body by utilizing nonlinear propagation distortion caused by harmonics occurring during ultrasonic wave propagation are well known in the art. In such a system, a transducer is driven alternately by a first and a second drive pulse of A and 2A, respectively, in amplitude. The transducer responsively transmits a first and a second ultrasonic wave, which are reflected by tissue in the body and received by the transducer as a first and a second ultrasonic echo of B and 2B, respectively, in amplitude. The first and second echoes are amplified by a variable gain amplifier with gains of C and C/2 to yield a first and a second signal of Bxc2x7C and 2Bxc2x7(C/2), respectively. Since the sidelobes of the first and second echoes are much smaller than the main lobes and accordingly small in distortion, the amplitudes of the sidelobes of the second echo are substantially twice those of the sidelobes of the first echo. Thus calculating the differences between the first and second signals, i.e., Bxc2x7Cxe2x88x922Bxc2x7(C/2) enables the detection of the depth of reflection point. Since a pair of pulses is used for each analysis, such systems as described above are called xe2x80x9ctwo-pulsexe2x80x9d systems. A first and a second pulse in such a two-pulse system are hereinafter referred to as a xe2x80x9cformer pulsexe2x80x9d and a xe2x80x9clatter pulsexe2x80x9d.
However, in order for the above imaging technique to work satisfactorily, the reflection points or ultrasonic wave transmission directions from which the former and later echoes are obtained must be substantially the same. This restriction prevents high-speed scanning in conventional nonlinear distortion-based ultrasonic diagnostic imaging system.
In light of the above, it is an object of the present invention to provide a nonlinear distortion-based ultrasonic diagnostic imaging system which displays a raised-resolution video of tissue inside a body at an increased frame rate.
According to an aspect of the invention, a transducer transmits a ultrasonic wave pulse in response to a driving pulse while scanning the transmission direction in response to a scan control signal and receives an echo of the ultrasonic wave pulse to provide an echo signal. A transducer driver supplies the driving pulses and the scan control signal to the transducer such that the transducer transmits weaker and stronger ultrasonic wave pulses alternately while putting the same intervals between adjacent ultrasonic wave pulses to obtain a weaker echo of the weaker ultrasonic wave pulse and a stronger echo of the stronger ultrasonic wave pulse from the transducer. An equalizer equalizes each weaker echo to the stronger echo into an equalized weaker echo. An interpolator calculates an interpolation value between the equalized weaker echo and an equalized previous weaker echo obtained from a previous weaker echo. For each weaker ultrasonic wave pulse, a detector finds a value indicative of a difference between the interpolation value and a stronger echo obtained between the weaker echo and the previous weaker echo. An image processor generates a raised-resolution video signal of the tissue at an increased frame rate on the basis of the values and the scan control signal.
In one embodiment, the equalizer calculates a convolution by using each weaker echo as one of two components.
In the embodiment, the transducer driver may supply a narrower driving pulse and a wider driving pulse for the weaker and stronger ultrasonic wave pulses, respectively. Alternatively, the transducer driver may supply fewer driving pulse(s) for the weaker ultrasonic wave pulse and supply more driving pulses for the stronger ultrasonic wave pulse. These driving pulses have an identical width.
In the embodiment, the interpolator calculates an arithmetic means of said equalized weaker echo and said equalized previous weaker echo. Alternatively, an arithmetic means of the absolute values of the equalized weaker echo and the equalized previous weaker echo may be calculated.